1. Field of the Invention
The present invention relates to a hologram apparatus and a hologram recording and playback method.
2. Description of the Related Art
Among hologram recording media adapted to record digital data as a hologram is a medium that has a photosensitive resin (e.g., photopolymer) sealed between glass substrates. To record digital data on a hologram recording medium as a hologram, a coherent laser beam from the semiconductor laser device is first split into two laser beams with a PBS (Polarization Beam Splitter). Then, two laser beams, one (hereinafter referred to as “reference beam”) and the other (hereinafter referred to as “data beam”) reflecting the information of two-dimensional gray image pattern formed in an SLM (Spatial Light Modulator) obtained as a result of applying the other beam to the SLM having digital data formed as the two-dimensional gray image pattern, are applied to the hologram recording medium at a given angle. This causes the target digital data to be recorded in the hologram recording medium.
More specifically, the photosensitive resin making up the hologram recording medium has a finite number of monomers. When the laser beam (hereinafter referred to as “laser beam”) made up of the reference and data beams is irradiated thereinto, the monomers change into polymers correspondingly with the energy determined by the light intensity of the laser beam and the irradiation time. As a result of the transformation of the monomers into polymers, an interference fringe, made up of polymers, is formed correspondingly with the laser beam energy. Moreover, as a result of the formation of such an interference fringe in the hologram recording medium, digital data is recorded as a hologram. Later, remaining monomers migrate (spread) to those locations that have consumed monomers. Further, as a result of the irradiation of the laser beam, such monomers change into polymers. It is to be noted that FIG. 8 schematically illustrates how monomers transform into polymers correspondingly with the laser beam energy in the hologram recording medium.
It is also to be noted that if a large amount of digital data must be recorded in the hologram recording medium, the incidence angle of the reference beam into the hologram recording medium is changed to enable the so-called “angle-multiplexed recording” adapted to form a number of holograms. For example, a hologram formed in the hologram recording medium is called a page, whereas a multiplexed hologram made up of a number of pages is called a book. FIG. 9 schematically illustrates the book and the pages in the angle-multiplexed recording. As shown in FIG. 9, the incidence angle of the reference beam is varied to form, for example, ten pages of holograms for a single book in the angle-multiplexed recording. Thus, the angle-multiplexed recording allows for the recording of a large amount of digital data.
To play back digital data from the hologram recording medium, on the other hand, the reference beam is irradiated onto the interference fringe representing the digital data at the same incidence angle as when the interference fringe was formed. The reference beam (hereinafter referred to as “playback beam”) diffracted by the interference fringe is received by an image sensor or other means. The playback beam received by the image sensor or other means constitutes a two-dimensional gray image pattern representing the above-described digital data. Then, the digital data can be demodulated from this two-dimensional gray image pattern with a decoder or other means to play back the digital data.
It is to be noted that FIG. 1 in Japanese Patent Application Laid-open Publication No. 2004-177958, for example, discloses a hologram recording/playback apparatus operable to record holograms to and play them back from a hologram recording medium as described above.
Incidentally, a photosensitive resin, a resin that changes its molecular structure as it receives a laser beam, is primarily used as a hologram recording medium as described earlier. It is known that a hologram recording medium varies in volume due to this reaction that causes the photosensitive resin to change its molecular structure. Further, this volume change is known to vary depending on the characteristics of the hologram recording medium and the light intensity of the laser beam. For this reason, even if a laser beam is applied to a hologram (interference fringe) recorded in the hologram recording medium at the same incidence angle as during the hologram recording, the hologram may not be played back properly. FIG. 10 illustrates how a hologram recording medium contracts as an example of the volume change of the hologram recording medium.
In FIG. 10, the incidence surface refers to one of the surfaces of the hologram recording medium struck by the laser beam, whereas the vertical direction refers to the normal direction at an arbitrary point or the latitudinal direction of the hologram recording medium assuming that the incidence surface is a curved surface, and the horizontal direction refers to the direction vertical to the normal direction and the longitudinal direction of the hologram recording medium.
Here, the contraction of the hologram recording medium is dispersed in the horizontal direction. Therefore, the shape thereof will not undergo much change. In the vertical direction, on the other hand, the region of the medium is limited. This makes the dispersion of the contraction small. As a result, the shape change tends to be large. If such a contraction takes place, the disposition angle of the interference fringes changes although the spacing between interference fringes, recorded in the hologram recording medium, does not vary much. This change in the interference fringe disposition angle leads to a discrepancy between the incidence angle of the laser beam during the hologram recording and that during the hologram playback. It is to be noted that, in the example shown in FIG. 10, the laser beam is first applied to the hologram recording medium at the incidence angle of θr relative to the vertical direction before the contraction. After the contraction, on the other hand, the laser beam is applied to the hologram recording medium at the incidence angle of θs relative to the vertical direction. To play back the hologram after the contraction, therefore, the laser beam, corrected to the angle changed as a result of the contraction, must be applied to the hologram recording medium. For this reason, a method is proposed, that consists of searching for a proper laser beam incidence angle while at the same time changing the laser beam incidence angle onto the holograms recorded in the hologram recording medium to play back the holograms, as a simple and reliable method. However, this method requires several hologram playbacks before the proper laser beam incidence angle can be searched for. This makes the method unfit for the hologram playback that is also demanded to be faster.